When humans think about sex, we tend to categorise into male and female forms, says biologist and animal sex expert Carin Bondar. But for millions of years, it used to be a fusion of bodies, a “trickle of DNA shared between two or more beings,” she says. It wasn’t until about 500 million years ago that we developed structures akin to the penis, which delivers DNA, and the vagina, which receives it.

Pretty neat, but it’s much more complex than that. In fact, the diversity of sexual structures evolved for reproduction is utterly mind-blowing. “Penile diversity is especially profuse,” says Bondar. The paper nautilus, for example, has a hectocotylus — a detachable penis that swims over to the female and attaches itself to her nether regions, providing sperm. For decades, biologists thought it was an entirely different organism! The South American tapir has a penis that is fully prehensile and quite dextrous. It bypasses the female’s vagina to deliver sperm directly into her uterus. And then there’s the beach barnacle, who holds the enviable record for the world’s biggest penis-to-body ratio — 50 times the length of its entire body.

With all this structural diversity, says Bondar, you’d think male and female parts would fit easily, but it’s not that simple. Why? It’s not just about form but also function — the contributions made by the gametes are far from equal. Male and female sexual strategies differ because eggs are energetically expensive to produce, so females are choosy about mating, while sperm are abundant and cheap, and males — wanting to give their genetic material a chance — go with “more sex better.” Nature copes with these different agendas by making sexual organs mismatched enough that it requires effort for sperm to get to egg.

Photo: James Duncan Davidson

Male bedbugs are aggressive about it: they have a spiked, barbed penis, with which they stab into their partner’s body cavity. The sperm migrates to the ovaries. If a female gets too many wounds, she can actually die. Aptly, this is called “traumatic insemination.” Male ducks essentially gang-rape females — cornering her and holding her down and, with their corkscrew shaped penises, ballistically ejecting their sperm into her corkscrew-shaped vagina. The female still has some say, though. She can favor the offerings of a male by manipulating her posture, giving his sperm better access to her ovaries.

But it’s not all so barbaric. While the orgasm didn’t evolve until 65 million years ago with the appearance of mammals, pleasure counts for other animals, too. Male earwigs can either have very large or small penile appendages, and females choose the former. Why? One study posits that the longer penis stimulates the female’s reproductive tract prior to copulation — and females seem to enjoy it. Male Mexican guppies have a mustache-like structure whose purpose appears to be to prod the females’ genitalia — what Bondar lovingly calls the “Magnum PI hypothesis.” Females are much more likely to be found near males with large mustaches.

It would appear that the two strategies for reproduction appear to be male-instigated coercion and titillation, says Bondar, of which coercion is far more common in the animal kingdom. But power doesn’t always rest with the male. The females of two mammalian species — hyenas and elephants — have an external penile clitoris that covers the reproductive organs, which must be retracted in order for copulation to take place. In other words, no means no — if the female is not on board, he’s not getting any. Interestingly, both elephant and hyena societies live in matriarchal societies, run by females and groups of females — social structure mirroring sexual structures. In hyena societies, adult males are the lowest on the totem pole. “It seems that if you take away penis power from a male, you take away all the power he has,” she says.

“Sexual strategies and reproductive structures dictate how males and females react to each other, which dictate how societies evolve,” concludes Bondar. “What might surprise you is the extent to which so many other aspects of their — and our — lives are influenced by it.”

Horses, parrots and mosquitos: oh my! TED2013 will commence in just nine days, and the TED Blog is gearing up for our live, minute-by-minute, coverage. One thing we can’t wait to see: which speaker will bring an animal onstage with them? (Our money is on ornithologist Kees Moeliker.) Sometimes, speakers arrive with a live animal, […]

It is Dead Duck Day 2015 — the 20th anniversary of the day in 1995 when a mallard duck flew into a glass window outside the Natural History Museum Rotterdam, fell to the ground dead, and then became part of the first scientifically observed instance of homosexual necrophilia in mallard ducks. Now, each June 5, curator Kees Moeliker […]